sm: implement in-band TLS upgrade for RFC 3588 backward compatibility

[tridentsx]

sm: implement in-band TLS upgrade for RFC 3588 backward compatibility

Summary

Re-implements post-CER/CEA TLS upgrade via Inband-Security-Id negotiation, with corrected RFC citations and fixes for the race conditions identified in the original review.

This is a backward-compatibility mechanism for interoperating with RFC 3588 peers that do not support TLS-before-CER (port 5658). RFC 6733 explicitly retains this capability.

Zero impact when unused

If no peer sends Inband-Security-Id=1 in CER, or if Settings.TLSConfig is nil, the new code paths are never entered. The existing behavior is completely unchanged — peers that don't request in-band TLS are unaffected.

RFC 6733 basis

§1.1.3 — Changes from RFC 3588:

This new approach augments the existing in-band security negotiation, but it does not completely replace it. The old method is kept for backward compatibility reasons.

§5.6 — Peer State Machine (p.69):

When connecting to responders that do not conform to this document (i.e., older Diameter implementations that are not prepared to received TLS/TCP and DTLS/SCTP connections in the closed state), the initial TLS/TCP and DTLS/SCTP connection attempt will fail. The initiator MAY then attempt to connect via TCP or SCTP and initiate the TLS/TCP and DTLS/SCTP handshake when both ends are in the open state.

§13 — Security Considerations (p.140):

For TLS/TCP and DTLS/SCTP connections to be established in the open state, the CER/CEA exchange MUST include an Inband-Security-ID AVP with a value of TLS/TCP and DTLS/SCTP. The TLS/TCP and DTLS/SCTP handshake will begin when both ends successfully reach the open state, after completion of the CER/CEA exchange. If the TLS/TCP and DTLS/SCTP handshake is successful, all further messages will be sent via TLS/TCP and DTLS/SCTP. If the handshake fails, both ends MUST move to the closed state.

§6.10 — Inband-Security-Id AVP:

The use of this AVP in CER and CEA messages is NOT RECOMMENDED.

Note: NOT RECOMMENDED per RFC 2119 §4 means "there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful" — it is not a prohibition.

Prior discussion

This was discussed in #238 where @vingarzan (who raised the original RFC compliance concern) concluded:

"if there is even a single stack which does that and you need to interconnect with it, of course, you must go for it. [...] If it is RFC6733 compliant too, it's not affecting me or anyone else, of course."

The previous PR (#243) was reverted due to an incorrect §6.2 citation. This resubmission uses the correct references throughout.

What's included

1. TLSUpgrader interface — StartTLS / StartTLSClient on Conn for upgrading plain connections after CER/CEA.

2. Server-side CER handling — Accepts Inband-Security-Id=1 when Settings.TLSConfig is set. Performs TLS upgrade after sending success CEA.

3. CEA echoes Inband-Security-Id=1 — Per §13, both peers must agree on the security mechanism.

4. Client-side CEA handling — After receiving a success CEA, the client calls StartTLSClient to complete the upgrade.

5. Race condition fix: bufio drain — The bufio.Reader may have pre-read the TLS ClientHello. Before handing the connection to tls.Server, buffered bytes are drained and prepended via io.MultiReader.

6. Race condition fix: synchronous CER dispatch — CER (command code 257) is always dispatched synchronously regardless of MaxConcurrentHandlers, ensuring the TLS upgrade completes before the serve loop reads the next bytes.

Test coverage

Function	Coverage
StartTLS	100%
StartTLSClient	100%
startTLS	83%
handleCER	81%
handleCEA	86%
successCEA	97%
dispatch	100%

Tests:

• TestHandleCER_InbandSecurity_WithTLSConfig — server accepts CER with Inband-Security-Id=1 when TLSConfig is set
• TestInbandTLSUpgrade_EndToEnd — full plain TCP → CER/CEA → TLS upgrade → DWR/DWA over TLS
• TestConcurrentServePanicRecovery — updated to use DWR since CER is now synchronous
• go test ./... passes (SCTP test excluded — requires kernel SCTP support)